Progressive die apparatus is typically employed for manufacturing laminated iron cores for rotating electric machines from a strip of electromagnetic steel sheet in coil form (sheet steel strip). In the progressive die apparatus, iron core laminates are produced from the sheet steel strip by sequentially punching out pilot holes, slots and inner teeth and blanking the individual iron core laminates. A prescribed number of iron core laminates are stacked and fixedly attached to one another to finally produce the iron cores.
The iron core laminates that are blanked by the blanking punch are pushed into a squeeze ring placed under the die one by one, and are laminated and stacked together while receiving a side pressure from the squeeze ring which has an inner diameter substantially equal to or slightly smaller than the outer diameter of the iron core laminates. The methods for fixedly attaching the iron core laminates with one another include the lamination crimping method where each individual iron core laminate is formed with a crimping feature so that the adjoining iron core laminates are crimped to one another as the iron core laminates are laminated and stacked, the laminating bonding method where a bonding agent is applied to a surface of the sheet steel strip in the progressive die apparatus, and the iron core laminates are bonded to one another simultaneously as the iron core laminates are laminated and the laser welding method where the blanked iron core laminates are laser welded to one another.
To more effectively stack the iron core laminates in the squeeze ring, various back pressure devices have been produced. A support table is placed under the squeeze ring (ejecting end of the laminated iron core) to apply an upward pressure (back pressure) on the lower surface of the lowermost iron core laminate. For instance, in the lamination crimping method, the back pressure on the iron core laminates may be increased for increasing the crimping mechanical strength. However, if the increased back pressure exceeds the retaining force for the iron core laminates created by the side pressure, the iron core laminates in the squeeze ring (or in the die) may be pushed upward by the support table.
To overcome such a problem, it was proposed to provide a vertically moveable base table, an elastic member (such as a gas spring) placed on the base table and a support plate attached to the upper end of the elastic member to stack iron core laminates thereon such that the vertical position of the base table may be fixed during the downward stroke of the punch. The elastic member is compressed by the support plate which is pushed downward by the downward stroke of the punch, and applies a force that counteracts the pressure from the punch to the support plate so that the back pressure may be more effectively applied to the iron core laminates. See Patent Document 1.